Trump has been pushing NATO members to reach their agreed-to target of spending 2 percent of their gross domestic products on national defense by 2024 and has accused those who don't of freeloading off the U.S.

NATO estimates that 15 members, or just over half, will meet the benchmark by 2024 based on current trends.

During his campaign, Trump called NATO "obsolete" and suggested the U.S. might not come to the defense of members if they found themselves under attack — a shift that would represent a fundamental realignment of the modern world order. He also called Brussels a "hell hole" and "a mess." Trump has moderated his language somewhat since taking office, but has continued to dwell on the issue, even as many NATO members have agreed to up their spending.

Stoltenberg, for his part, credited Trump for spurring NATO nations to spend more on defense, noting that the Europeans and Canada are projected to spend around $266 billion more by 2024.